The invention relates to catalysts useful for olefin polymerization. In particular, the invention relates to xe2x80x9csingle-sitexe2x80x9d catalysts that incorporate one or more chelating ligands derived from pyrimidines.
While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry""s future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of xcex1-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes incorporate one or more cyclopentadienyl (Cp) or Cp-like anionic ligands such as indenyl, fluorenyl, or the like, that donate pi-electrons to the transition metal. Non-metallocene single-site catalysts have evolved more recently. Some of these include pi-donor heterocyclic ligands that are isolobal to the cyclopentadienide anion, such as boraaryl (see U.S. Pat. No. 5,554,775) or azaborolinyl (U.S. Pat. No. 5,902,866). A different type of non-metallocene single-site catalyst capitalizes on the chelating effect. Two or more sigma-donor atoms coordinate to a transition metal in these complexes. Examples are 8-quinolinoxy or 2-pyridinoxy ligands (see U.S. Pat. No. 5,637,660), the bidentate bisimines of Brookhart (see Chem. Rev. 100 (2000) 1169), and the tridentate pyridine bisimines (see J. Am. Chem. Soc. 121 (1999) 8728). The bisimines are valuable for use with late transition metals (Groups 8-10).
Interest in late transition metal catalysts for polyolefin manufacture is growing, largely because of their potential for high activity and their ability to tolerate and incorporate polar comonomers. Recently, we described catalysts based on late transition metals and aromatic 1,2-diimine ligands derived from 1,2-diaminobenzenes (see copending application Ser. No. 09/711,364, filed Nov. 10, 2000). While the synthesis of these ligands avoids the need for an xcex1-diketone starting material (used to make Brookhart""s complexes), the aromatic 1,2-diimines have some disadvantages. For example, steric crowding may limit yields of the diimines. Moreover, unwanted side reactions, such as the reaction of two amino groups with one mole of aldehyde or ketone to give a benzimidazole in an intramolecular cyclization reaction, are documented (see U.S. Pat. No. 4,427,802, Example 5).
In spite of their versatility, pyrimidines have apparently not been used as ligands for olefin polymerization catalysts. They have been described as antioxidant moieties for polyolefin stabilizers but not as catalyst components. Nonetheless, pyrimidines occur in nature and have been known for more than a century. Moreover, they are easily synthesized with well-known routes and many are commercially available.
There is a continuing need for single-site catalysts that can be prepared inexpensively and in short order from easy-to-handle starting materials and reagents. In particular, there is a need for new ligands that can be used with late transition metal complexes. Ideally, the ligand structure could easily be modified to provide control over catalyst activity and polyolefin properties.
The invention is a catalyst system useful for polymerizing olefins. The catalyst system comprises an optional activator and an organometallic complex. The complex incorporates a Group 3 to 10 transition metal and at least one neutral or anionic chelating pyrimidine ligand. Chelating pyrimidines are easy to make, and they are readily incorporated into transition metal complexes, including those based on late transition metals. By modifying the chelating groups and other substituents on the pyrimidine ring, polyolefin makers can increase catalyst activity and control polymer properties.